PROJECT SUMMARY/ABSTRACT There are over 34 million people currently infected with HIV-1 worldwide and at least 2 million new infections each year. Given this high prevalence and the associated economic and social toll, a prophylactic vaccine protective against HIV-1 is of paramount global health importance. In general, successful anti-viral vaccines induce the production of antibodies able to bind the pathogen and block infection of cells. While antibodies able to bind and broadly neutralize diverse strains of HIV-1 have been characterized in natural infection, tested candidate HIV-1 vaccines only induce antibodies protective against a narrow subset of strains closely related to the virus the vaccine was derived from. While several hypotheses have been proposed, it is not understood why vaccines fail to induce antibodies able to bind the conserved regions of the virus targeted by protective antibodies that develop in some chronically infected individuals. To understand the mechanism(s) preventing the production of these protective ?broadly neutralizing? antibodies after vaccination, we will study the activation and differentiation of the cells responsible for antibody production, B cells. To date, all studies of HIV-specific B cells have occurred following vaccination or infection. This is because the ?naive? HIV-specific B cells present before vaccination or infection are extremely rare and difficult to identify. To overcome these limitations, we have developed a novel enrichment and analysis strategy to identify naive HIV-specific B cells. This approach allows for the direct ex vivo analysis of human HIV-specific naive B cells, allowing for an unprecedented pre-clinical analysis of the cells targeted by candidate vaccines. This innovate approach will reveal factors limiting the activation of HIV-specific naive B cells, which will inform the design of new vaccines that overcome these deficiencies.